Coho Salmon

-->Salmon are characterized by their unique life histories that involve life in both fresh and marine waters. Though born in freshwater streams, adult salmon spend the majority of their lives in the open ocean. Once they reach sexual maturity, they return to the streams of their birthplace to spawn.

            Coho salmon in particular migrate to their stream of origin to spawn, around the age of three years old. Spawning males change dramatically in appearance, developing a sharply hooked snout and large teeth, and darkening to a deep red. Female coho spawners also darken, and, upon mating, dig nests called redds into the stream substrate where they lay their eggs.

From top: Adult coho ocean form, male spawner, female spawner, fry [2].

Coho salmon spawn only once, and then die, their carcasses fertilizing the stream environment and providing nutrients for developing salmon larvae. The eggs hatch in 6-7 weeks, and the tiny alevin larvae live in the gravel and feed off of their yolk sac for several weeks. At 5-10 weeks the small salmon are called fry and are active and swimming, feeding on plankton and small insects. Growing parr, characterized by their camouflage markings, retain a pattern of vertical bars and spots until the reach the appropriate size for migration to the ocean. At about a year old, these migrating smolt lose their parr markings and take on the dark back and light belly of fish in the open ocean. Their gills and kidneys also begin to change to accommodate salt water. Traveling in groups, the smolt reach the sea where they spend the next two years as adults feeding on small fish [4] [5] [6].

 

North American distribution of coho salmon [3].

 The distribution of Coho salmon stretches from Alaska to central California, with populations found in most coastal areas. While Alaskan stocks of Coho are generally healthy and productive, some populations of Oregon and California have been listed as threatened and endangered. The Puget Sound population of coho has also been listed as a species of concern. Listed populations have been experiencing an ongoing decline in numbers, which warrants increasing attention to the contributing factors of this trend, as well as what can be done to combat it [5].

The factors affecting salmon population are numerous and compounding. Due to both human-induced and natural variables, the abundance of salmon has plummeted in the past several decades, complicated by the intricate life histories of salmon species and their utilization of multiple habitats throughout the life cycle. Human modification of waterways for agriculture, hydropower, and flood control has restricted accessible habitat and migration paths, altering water flow and consequently temperature, sediment composition, and critical migration flow levels. Dam turbines also result in increased mortality of migrating juveniles and returning adults, despite the creation of fish ladders to circumvent this problem. Further, habitat fragmentation and deconstruction arises from land use associated with continued logging and urban development, ultimately decreasing the availability of suitable habitat for spawning. In addition, and exacerbating human-related population decline, an increase in hyper-conditions of the natural environment, such as floods and drought, further limit the availability of undisturbed habitat. Similarly, climate-induced decreases in ocean productivity create challenges to population sustainability due to trophic limitations in the food chain. These climatic shifts have come under intense scrutiny for their role in accelerating the decline of salmon populations to critically low levels. It is the cumulative effect of climate change that we focus on here, both the potential repercussions for coho salmon populations and the ways in which we humans might respond in our responsibility to preserve salmon as a critical northwest element [7].